The proposed research will include the following: (1) Ongoing studies of chemical modification of allosteric sites of rabbit skeletal muscle phosphofructokinase (PFK) will be extended. These stuides will examine the kinetic regulatory properties of PFK modified at the AMP binding site and of PFK hybrids containing native enzyme and enzyme with either a modified reactive thiol group, a modified citrate binding site, a modified ATP inhibitory site, or a modified AMP site; (2) A study of the characterization, distribution, and physiological role of multiple isozymic forms of PFK will be continued employing electrophoretic and immunochemical techniques. Emphasis will be placed on the yet to be characterized C isozyme and its regulatory properties. In addition, we will examine possible adaptive responses by brain, liver, and adipose tissue PFKs to varying dietary and hormonal states; and (3) The regulatory significance of phospho- and dephospho-forms of muscle PFK will be investigated. Immediate goals for this project are: 1) To determine structurally important features of PFK using partially proteolyzed forms. 2) To seek correlation between the extent of phosphorylation of PFK and hormonal and functional changes in muscle. 3) To determine differences in activity and/or stability in phosphorylated and non-phosphorylated enzymes.